This invention relates to a process of supplying heat to chemical reactions, particularly to a cracking of hydrocarbons for producing gas, by a combustion of fuels with heated combustion air and an extraction of sensible heat from the flue gases produced by the combustion, which extraction is carried out in a heat exchanger (air preheater) for heating the air intended for combustion.
When liquid or gaseous fuels are industrially burnt to produce steam or to maintain chemical processes, difficulties always arise because the temperature of the flue gases decreases locally below the dew point, particularly when sulfur-containing fuels are burnt. When sulfur-free fuels are used for heating, the condensation products contain small amounts of carbon dioxide and oxygen and only slightly attack the steel parts of the plant which are contacted by them. A different situation arises when the temperature of flue gases produced by the combustion of sulfur-containing fuels decreases below the dew point. The sulfur contained in the fuel is oxidized by the combustion to SO.sub.2 and SO.sub.3 and these compounds increase the water vapor dew point. The increase of the dew point depends mainly on the sulfur content of the fuel. Additional factors are the excess of air present during the combustion and the nature of the fuel.
The water vapor dew point of the flue gases produced by the combustion of sulfur-free gasoline is about 50.degree.C, and the dew point of the flue gases produced by the combustion of light fuel oil (EL fuel oil), which contains 0.5 % sulfur, is about 80.degree.-85.degree.C. A dew point of about 160.degree.-180.degree. C must be expected when heavy fuel oil which contains about 4% sulfur is burnt. If the temperature in the flue gas decreases below the dew point, the condensation product will contain sulfurous acid and sulfuric acid in different concentrations and will very strongly attack most metallic materials. Owing to the danger to heat exchangers of steel, exhaust fans and steel chimneys, it is often necessary to feed the flue gases at relatively high temperatures into the exhaust gas chimney so that temperatures below the dew point will be reliably avoided.
In the conventional arrangements of heat extractors in a fired system, the sensible heat, which is at a high temperature, is utilized in most cases for a superheating of steam, for a heating of chemical processes, or for preheating process feedstocks. In some cases, the highest peak temperature peaks are taken up by a steam producer, which is preferably operated in the range above about 250.degree.-300.degree.C. Special consideration will then be required if the sensible heat of the flue gases below said temperatures is to be utilized further. In this connection, the corrosion problem must also be taken into account. If, for this reason, the flue gases are supplied into the exhaust gas chimney at an excessively high temperature, considerable energy will be lost without utilization.
Combustion air preheaters are generally used for a further utilization of the sensible heat of the flue gases. The preheating of the combustion air increases the economy of the fired system. The heat exchangers for preheating the combustion air are made in most cases from steel or cast iron although these are attacked by acids. Whereas cast iron resists concentraded sulfuric acid, it is attacked by an acid of lower concentration. This fact may necessitate the use of glass tubes for protecting parts of the air preheater which are endangered by corrosion. In that case, however, in a tube bank heat exchanger the side where the cold flue gases exit and the side where the cold combustion air enters as well as the flue gas duct leading to the exhaust fan and the fan itself remain endangered.
Hot flue gas and cold combustion air on both sides of a conventional combustion air preheater have approximately the same film coefficient of heat transfer because these two gases under low pressure are caused to flow approximately at the same velocity. As a result, the tube wall temperature in the heat exchanger is approximately midway between the temperatures of the flue gas and the combustion air. Owing to the poor film coefficients of heat transfer, a strict counterflow arrangement or a cross-counterflow arrangement is selected in most cases in order to minimize the dimensions of the heat exchanger and to provide for an air temperature which is as high as possible. If the mean exit temperature of the flue gas is 150.degree.C, and the air entering the heat exchanger is at a temperature of 25.degree.C, a tube wall temperature of about 87.degree.C must be expected in a counterflow arrangement. When EL fuel oil is used, that temperature is still above the dew point of the resulting flue gases. On the other hand, if the entering air is at a temperature of 0.degree.C, the tube wall temperature will be only about 75 .degree.C so that a condensation in the flue gas and consequent corrosion must be expected. As the air temperature decreases, more condensate is formed and the corrosion increases. In heat exchanger providing for a cross-counterflow and fed with flue gases having the same average exit temperature, the local temperature at the air entrance will be lower by as much as 80.degree.C so that temperatures below the dew point must always be expected.